This invention is concerned generally with broad band amplifiers, and, more particularly, with automatic gain control (AGC) amplifiers.
A broad band, variable gain, AGC amplifier typically comprises a differentially connected pair of matched transistors as a basic circuit around which the various circuit configurations of the prior art are constructed. The input signal to the differentially connected pair is the sum of the emitter currents of the two transistors and the output signal is the collector current of one of the two transistors. Typically, the inverse of the amplifier current gain is an exponential function of the differential base bias voltage of the differentially connected transistor pair plus unity. Various methods of controlling the differential base bias voltage have been used in the prior art, but none of these techniques provides the ability to vary the amplifier gain hyperbolically as a function of a D.C. control signal. Also, the differentially connected transistor pair without compensation typically has a temperature-dependent gain which is an exponential function of a selected control signal.
In most applications, the output signal of the AGC amplifier is to be kept constant for all levels of the input signal. The basic differentially connected transistor pair suffers from two major disadvantages in these applications. The first disadvantage is that the modulation index of the output signal is equal to the modulation index of the input signal and both vary widely. The modulation index is defined as a ratio of the A.C. signal current to the D.C. bias current. The variation in the output modulation index has not been solved -- only tolerated -- by the prior art. In a paper by Barrie Gilbert entitled "A New Wide-Band Amplifier Technique", IEEE Journal of Solid State Circuits, Vol. SC-3, No. 4, pp. 353-365, December 1968, a four-quadrant multiplier was introduced which eliminates the variation of both the input and output D.C. bias currents but not the variation of the modulation indices. In addition, this realization is considerably noisier than the basic differentially connected transistor pair.
The second disadvantage is that the r.m.s. noise component of the output current is proportional to the D.C. component of the input current, assuming that the dominant noise source is the base resistance of the transistors in the differentially connected pair. In the paper by W. M. C. Sansen and R. G. Meyer, entitled "An Integrated Wide-Band Variable-Gain Amplifier with Maximum Dynamic Range", IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 4, pp. 159-166, August 1974, a constant current is subtracted from the input D.C. bias current by connecting a resistor between the positive voltage source and the differentially connected emitters of the transistor pair, forcing the differentially connected pair amplifier to operate at a lower D.C. bias current. This solution, however, does not optimize the noise performance over the range of circuit gains and is only of value when the modulation index of the input signal is sufficiently small, which is not always the case.